The primary objectives of an uninterruptible electric power supply system are to: (1) sense normal power supply interruption, failure or inadequacy, (2) upon interruption, switch electric power supply to a standby and/or emergency supply, and/or switch loads from normal to emergency or limited loads (within the capability of the stand-by or emergency supply), (3) provide long term standby or emergency power until normal power supply is again available, and (4) provide transitional power and control during the period between loss of normal power and stand-by or emergency supply. Other useful functions include removing of unwanted spikes or transients from the normal power supply, power-factor correction, and monitoring/alarm to notify of the loss and/or operation of the uninterruptible supply.
The uninterruptible power supply should be small so that it will not occupy valuable space or interfere with normal functions, capable of convenient testing to verify emergency capability, be able to operate for extended periods of failure of the normal power supply and low in cost. It should also be light weight, rugged in construction, and low in cost. When the system is used to remove transients or during testing, a minimum of effort to convert from one mode of operation/test to another mode is also desirable.
Most of the current uninterruptible power supplies may do one of these objectives well, but others poorly or not at all. These limitations may be acceptable in many applications, but are unacceptable in some critical applications, such as hospitals or large computer installations. Many of these critical applications have multiple standby and/or emergency systems to achieve acceptable performance and reliability.
One type of uninterruptible power supply uses a large motor generator set with a large flywheel. This can also function as normal (electric power only or cogeneration) as well as standby electric power. The flywheel provides a short (transition) period of energy storage when power is lost, allowing control and switching to a stand-by unit. A common variation of this approach is to add a motor to the engine generator set. The motor, supplied by normal power supply, such as a public utility, drives the generator under normal conditions. When normal power is interrupted, the kinetic energic stored in the flywheel provides sufficient transitional power until the engine (typically a combustion engine) can be started. The combination also isolates the load from the normal power supply's transients and shortens the transition time by eliminating the time required to bring the standby generator up to synchronous speed. However, power-failure sensing switch response and startup time are still significant, requiring substantial amounts of energy to be stored in the flywheel. Units tend to be very heavy because of the flywheel mass, complex and cumbersome, limiting transport, installation, access and use.
Sensing of power interruption and switching of these rotating uninterruptible power supply systems has been accomplished by power and/or current relays, current limiters, under and over frequency relays, leading/lagging power phase detectors, and excessive vibration detectors. These types of sensors/switches are capable of handling large currents/amperage, but are relatively slow, requiring the heavy flywheels or other measures to supply transient power. Resetting procedures for these devices can also create the need for extended storage even if normal power is restored, unless reset provisions are included.
Another approach is to provide battery storage. An inverter can be used to convert the dc battery power to ac power. A further modification allows continuous conversion and charging of the battery during normal operation and battery discharge during charging interruptions, also isolating the loads from transients in the normal power supply. A major advantages of the battery type of system is the quick response to normal power interruptions and reset provisions. However, battery systems become very large and costly for storing significant amounts of power for extended outages. They also suffer from round trip electrical power conversion losses (ac to dc to ac). energy. The output waveform of the inverter contains many spikes and harmonics which make it unsuitable for many applications.
Combinations of battery powered equipment and motor generator sets have also been used. These multi-component systems try to take advantage of the best aspects of battery storage (i.e.: quick response) with the best aspects of rotating equipment (i.e.: long term outage capability). Other combinations of storage and standby units are also known.
However, these prior multi-mode approaches have many limitations. These limitations are primarily related to the multiplicity of elements required to accomplish the operating modes, creating added cost, weight and space. This multiplicity of elements does provide some dual capability, but during many critical times, only one of the component subsystems can supply the electric load. This particularly adversely affects reliability that is a key goal, especially in critical applications. In addition, some of these combined systems require frequent and separate types of maintenance from two separate organizations.
What is needed is a rotating electric power supply system capable of quickly providing a transient as well as long term source of motive power without interruption to critical loads and without a massive flywheel or separate battery/power conditioning system.